In an image pickup apparatus using a solid-state image pickup element, an appropriate exposure is set by a light quantity adjustment unit such as a shutter, an iris aperture, a gain control, and an ND filter, and an image is obtained. In particular, in the image pickup apparatus that obtains movie such as a digital video camera, the light quantity adjustment is carried out by the ND filter due to the following reason.
That is, in a shutter speed or gain control, a light quantity adjustment range is largely restricted because of a request by a frame rate, and a depth of field changes at the same time as the light quantity adjustment in the iris aperture. For that reason, the shutter speed or gain control cannot be used in a case where the depth of field is intentionally set to be shallow, and also a fundamental problem occurs that a resolution power is decreased if a large F value is adopted. This becomes a large constraint particularly in the case of using an image pickup element having a small pixel pitch. Therefore, in the image pickup element that obtains the movie, the light quantity adjustment is most typically carried out by the ND filter. Furthermore, as compared with a case in which a filter having a plurality of fixed optical densities is arranged in a retractable manner on an optical path as in related art, it is simple to arrange a single filter that can arbitrarily change the optical density, and also a degree of freedom in terms of image formation is also high. For the above-mentioned ND filter, a light control element using liquid crystal or an electrochromic material is proposed up to now, but a commercialization of product as the light control unit in the image pickup apparatus does not take place yet due to reasons of a reliability, a durability, and the like.
Also, for the light control element, a light control element is proposed which can improve a response speed even in a low temperature environment where the response speed decreases.
PTL 1 discloses a technology of improving the response speed in the low temperature environment by installing a heating circuit that generates heat through a direct power distribution to a transparent electrode in an electrochromic display apparatus.
However, the configuration of the heating through the direct power distribution to the transparent electrode needs a complex circuit configuration and a complex operation sequence such as a level shift circuit or a switching circuit. Also, a problem occurs that a voltage operation is largely restricted in a case where the heating alone is desired to be carried out without the drive of a display element.
PTL 2 discloses a technology of obtaining a practical response speed even in the low temperature environment by installing a heater electrode that directly heats up a liquid crystal layer in the vicinity of a drive electrode of the liquid crystal layer in a liquid crystal lens.
As the drive electrode and the heater electrode are electrically insulated from each other in this case, the drive can be carried out mutually independently, but this is not sufficient enough in terms of heating efficiency. That is, in the electrochromic element, a rate controlling on the response speed is conducted through an electrochromic layer formed on the transparent electrode and an ion diffusion on a boundary face between a transparent ion conductive layer and the electrochromic layer. For that reason, it is possible to improve the response speed more efficiently by heating an area in the vicinity of the electrochromic layer via a material with a satisfactory heat conductivity as compared with the case of heating an opposing inter-electrode void via a material with the low heat conductivity. For that reason, according to PTL 2, problems exist in a heat conductive path and a heat conductivity of a material on the heat conductive path.